Noise detection circuit



Aug. 11, 1953 w. R. YOUNG, JR 2,648,765

NOISE DETECTION CIRCUIT Original Filed May 20, 1949 4 Sheets-Sheet 1 /NVENTOR A TToR/VEV m R. rol/NG, J

Aug. 11, 1953 W. R. YOUNG, JR

NOISE DETECTION CIRCUIT Original Filed May 20, 1949 4 sheets-sheet- 2 /M/EN TOR W.V R. y0u/v6, un

Aug. 11, 1953 I w. R. YOUNG, JR 2,548,765

NOISE DETECTION CIRCUIT Original Filed May 20, 1949 4 Sheets-Sheet 3 nf R. you/v6, d

A TTOIPNE V Aug. l1, 1953 w. R. YOUNG, JR

NOISE DETECTION CIRCUIT 4 Sheets-Sheet 4 Original Filed May 20, 1949 /NVENTOR n4 R. rou/VG, am @MM/Q,

ATTORNEY Patented ug. l1', 1953 NOISE DETECTION CIRCUIT William R. Young, Jr., Summit, N. J., assgnor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application May .20, 1949, Serial No. 94,448. Divided and this application July 14, 1949, Serial No. 104,625

11 Claims. (Cl. Z50-20) This is a division of application Serial No. 94,448, filed May 20, 1949, for an improvement in Radiotelephone Receiving Systems.

This invention relates to electric signaling systems and more particularly to methods and means for selecting an individual message signal wave from a plurality of such waves.

It is frequently desirable to receive at a number of geographically separated locations the same message signals, and to select from the received signals only that train of received signals which, at any instant, provides the best communication channel. Multiple reception of the same message signals is frequently employed in mobile radio communication systems and in systems in which the signals are transmitted through a variable transmission medium, such that signals received at one location may be subjected to considerably more attenuation than are signals received at a different location.

In such communication systems, it is generally believed to be desirable to select, at any instant, only one such train of received message signals, since if two or more such signal trains are combined, the phase relations of the combined currents may be such as to produce a detrimental or undesirable effect. Furthermore, such signal wave energies are usually accompanied to a greater or lesser degree by noise signal currents. Where such signal currents are directly combined, a more desirable train of signal waves may be impaired by the noise energies that accompany another and less desirable train of signal Waves with which it is combined.

It is accordingly one object of this invention to improve the means for selecting from a plurality of received message signals the train of message signals which, at any instant, is judged to be the most desirable.

t is also an object of this invention to provide, in a radio communication system, a means for receiving the message signal waves in which the effect of interfering noise currents is minimized, and in which the reliability of reception is increased.

It is a further object of this invention to provide means for continuously determining the amount of interfering noise currents that accompany the received message signal waves, and to choose between the received message signals in accordance with this determination.

In accordance with the present invention, the same transmitted message signal wave may be received at a number of geographically separated points, .and the noise energy that accompanies each train of the received signal waves is continuously determined. At each receiver location there is derived a unidirectional voltage, the amplitude of which is indicative of the level of the interfering noise waves that are received with the message signals at that location. These voltage indications are transmitted to a centrally located selection point. At this selection point a means is provided for continuously comparing the noise-indicating amplitudes of these received unidirectional Voltage signals, and for connecting to a utilization circuit the output of the message signal receiver which produces the most desirable unidirectional signal. Means are also provided at this centrally located selection point for canceling the effect of any induced longitudinal currents that may be added to these derived signals in the transmission paths between the receiver. locations and the selection point.

It is a feature of this invention that the continuous determination of the noise energies that accompany the received message signals is made by determining the energy level of thenoise currents in the frequency spectrum immediatelyv above and beneath, but not within, the signal frequency spectrum.

. It is also a feature of this invention that this noisek energy determination is made in such a manner that the different types of noise interference are weighted in much the same manner as their interference eifects are noted. by the human ear.

It is a further feature of this invention that the relative desirability of each received train of message signal waves is indicated by a unidirectional voltage signal, derived or otherwise generated electrical quantity or factor, for eX- ample, the amplitude of which changes with variation in the noise energy that accompanies the signal wave.

Still another feature of the invention is that the effects of longitudinal noise currents that may be induced in the connections over which such merit indicating signals are transmitted, between the points of reception of the noise energies and the centrally located selection station, are nulliiied. f

The manner in which the foregoing objects and features of the invention are realized may be best understood from the following description when considered in conjunction with the drawing in which:

Fig.'1 is a block schematic diagram of a radio receiver selection system in accordance with the l 3 invention when arranged to choose between the signal outputs of a plurality of receivers of the same message signal wave;

Fig. 2 is a schematic diagram illustrating primarily one of the noise detecting and evaluating units that are incorporated in a receiver selection system in accordance with the invention;

Fig. 3 is a schematic circuit diagram illustrating primarily one of the longitudinal balancing units that may be used in a selecting system in accordance with the invention for counteracting the effect of induced longitudinal noise currents on the merit indicating signal;

Fig. 4 is a schematic circuit diagram illustrating primarily a signal comparison and selecting circuit in accordance with the invention, for comparing the noise or merit indicating signals from the various message signal receivers, and for con-v necting to a utilization circuit the receiver output that is associated with the most desirable noise indicating signal;

Fig. 5 is an explanatory graph to which reference is made in the circuit description; and

Fig. 6 shows how Figs. 2 3 and 4 may be arranged to disclose the complete circuit of one branch Aof a selecting system which is arranged in accordance with the invention.

General description The invention will now be generally described as being incorporated in a system for selecting the most desirable one of four trains of the same message signal waves. It will be understood that, although the invention is described as being incorporated in a system for selecting the output of one of four message receivers, this number does not constitute a limitation on the invention, since systems embodying the invention may involve as few as two or be expanded to choose from among any reasonable number of such message receivers. Referring to Fig. l, the same message signal waves are received on the geographically separated antennas I, IU', IU, and I". Each of these antennas is connected to a radio receiver I2, I2', I2, and I2"', respectively, which receivers are substantially alike and may be adapted for the reception of either amplitudemodulated, phase-modulated, or frequency-modulated signal waves. Each receiver is preferably arranged such that the volume of its audio frequency output signal is substantially independent of the level of its received radio frequency signal waves, if the energy level of these waves exceeds a predetermined minimum value; but is determined principally by the intensity of the modulating signal, or level of modulation at the transmitter. Under these circumstances, the level of the audio frequency message signal Waves is substantially the same in the output circuit of each receiver I2, I2', I2, or I2 at any instant, if the received radio frequency signals exceed this minimum level. Noise interference energy that may be received by any one of the antennas I0, Iii', etc. will be added to or included in the received message signal waves, and such interference energy will be more prominent in the receiver output product as its ratio to the audio frequency message signal energy in the receiver output is increased. The audio frequency output of each receiver I2, I2', etc. is divided, and one portion of this energy from each receiver is transmitted over connections I4, I4', etc. to a control terminal or selecting point, where it may be connected by a receiver selector I6 to a utilization d circuit I8. The second portion of this output energy is supplied to a respective noise detector 2i), 2G', etc., which is preferably situated at the same location as its radio receiver l2, I2', etc. Noise detector 20 operates on this portion of the output `energy to extract from it only the noise energies that exist in the frequency spectrum immediately above and below, but not within, :the message signal band. It has been determined that, in general, the amount of noise in the voice frequency `band of interest is closely related to the amount of noise which occurs above and below ythat band. A measure of these latter quantities is therefore an indirect but satisfactory measure of noise in the message signal band of interest. This extracted energy is then weighted in a marmer which will presently be described before it is used to derive a unidirectional signal, the amplitude of which is indicative of the weighted amount of noise encountered in the previously mentioned portions of the frequency spectrum. rThe amplitude of this unidirectional signal is related to the amount of extracted noise energy, and in this described embodiment, it varies in inverse relation to the amount of this noise energy. It should be appreciated that this inverse relation between the amount of extracted noise and the amplitude of the derived signal need not necessarily be followed, and it should not be construed to be a limitation upon the invention. These unidirectional merit indicating voltage s-ignals from the noise detectors 20, 20', etc. are transmitted to the control terminal, or selection point, over connections 22, 22', etc. For general descriptive purposes, these connections 22, 22', etc. are shown as being separate from connections i, It', etc. However, as will be later described in connection with Fig. 2, the paired connections I4, 22 or I4', 22', etc. may actually consist of a conductor pair.

The control terminal, or selection point, will generally be, but need not necessarily be, geographically separated from the location of receivers I2 and noise detectors 20. The unidirectional merit indicating signal from each noise detector 20 is received by a respective longitudinal balancing unit 24, 2li', etc. at the control terminal. In Athis unit, the signal is converted from a balanced-to-ground signal to a single-ended currents which may have been added to it in the connecting path 22 are effectively eliminated. The signal output of each longitudinal balancing unit 24, 26', etc. yconsists of a unidirectional voltage, which has at any instant an amplitude that is representative of the weighted amount of noise energy that accompanies the audio frequency message signal energy in the output circuit of a respective receiver. Since, as was previously stated, the message signal energy in the output of each receiver is at any instant at the same level, it follows that a comparison of the noise energy indications, or merit indicating signals, for the receivers is in leffect a comparison of the signal-to-noise ratio of the respective receivers. This comparative function is performed in receiver selector I6 at the control terminal. For general descriptive purposes, this unit is functionally indicated in Fig. 1 as a mechanical switch arrangement comprising the main contactor 26 and switches 28, 23', 28", etc. In addition, a threshold circuit 36 and threshold switch 32 are functionally shown as entities separate from the receiver selector I 6. Actually, as will be noted in connection. ,With the description of Fig. 4, these latter units may form a portion of the receiver selector I6. Selector I6 is arranged such that it responds to the merit indicating unidirectional voltage of greatest amplitude as received from the various longitudinal balancing units 24, 24', etc. to actuate its contacter 26 and close the circuit of only one switch 28, 28', etc. at any instant. The closure of a switch 28, 28', etc. connects to the output circuit 34 the audio signal output of the radio receiver that produces the unidirectional merit indicating signal of lgreatest amplitude. Concurrently with this action, the merit indicating signal from each balancing unit 24, 24', etc. is supplied to the threshold circuit 30, in which it is determined whether any one of the signals exceeds a predetermined minimum value that has been chosen as being representative of just usable or minimum quality received signals. If any one of the merit indicating signals exceeds this minimum value, it is indicative that some one of the receivers I2 is producing in its output circuit, the desired message signals at a level which, with rsepect to any accompanying noise currents or signals, has been previously adjudged to be suitable for service purposes. Under these conditions, the normally non-operated threshold switch 32 is closed, and output circuit 34 is connected to utilization circuit I8. Utilization circuit I8 may be any suitable terminating circuit such as a subscribers subset, a connecting circuit such as a trunk circuit, or a switch-board termination. If none of the merit indicating signals exceeds this minimum, or just-usable value, the threshold switch 32 remains non-operated; notwithstanding that receiver selector I6 may have actuated its contactor 26 to select the best one lof the subnormal indicating signals which it received from balan-cing units 24, 24', etc., and, in so doing, has connected the signal output circuit of the appropriate receiver to the selector output circuit 34.

The manner in which these described func-A tions are performed may best be .understood from a consideration of Figs. 2, 3, and 4 when arranged in the fashion shown in Fig. 6. In Figs. 2 and 3, only one noise detector 20 and one longitudinal `balancing unit 24 are shown, inasmuch as it is believed that an understanding of this portion of the practice of the invention can readily be secured by a consideration of one selecting branch only. It should, of course, be appreciated that a complete selecting system would ordinarily employ as many detectors 20 and balancing circuits 24 as there are receivers I2 from which a selection is to be made. In Fig. 4, the details of two, the upper and lower, selecting channels only are shown, since it is believed that a thorough understanding of this portion of the invention may be secured from a consideration of the details and action of these two channels. It is to be understood that the circuit details of the intermediate selecting channels, the presence of which is diagrammatically indicated by the block representations, are substantially the same as those shown for the other channels.

Detailed description as seen through the transformer 3l, will not have .f

an appreciable effect on the audio frequency and noise voltages appearing on conductoror lead 36. At this point, it may be noted that conductor pair 33 forms the same conductive connections that were shown separately in Fig. 1 as connections I4 and 22. One portion of the signal output from receiver I2, constituting the message channel thereof, is transmitted over conductor pair 33 tothe receiver selector unit I6 (Fig. 4). A second portion of the output from this receiver, constituting the merit indicating signal channel thereof, is transmitted to noise detector ZIJ over conductor 36 to a frequency sensitive network comprising resistors 40, 42, 44 and capacitor 33. This network discriminates in. favor of the higher frequency noise energy, and aids in evaluating the interfering effect of unwanted interfering periodic radiations which may be modulated by low frequency components. One portion of the output from this network is connected over conductor 46 to a band-pass filter 48, the values of the circuit elements of which are so chosen that it rejects all frequencies within the band of message signal frequencies, and passes only frequency components both above and below this signal band. It is a purpose of this filter to eliminate, in so far as it is practicable, the message signal frequencies that are supplied over conductor 46. In a tested embodiment of the invention, this filter was designed to reject frequencies in the band between 200 and 7000 cycles per second and to pass or transmit frequencies outside of that band. Noise or interference frequency currents that are passed by filter 48 are connected through transformer 50 to the input of a variable loss circuit, which comprises transformers 50, 60, unidirectional conducting devices 52, 56, and capacitors 54, 58. Devices 52, 56 mayy be constructed of any of several suitable materials, and each has the characteristic that it conducts current in one direction with considerably greater facility than it does in the opposite direction, and also that its resistance in its conductive direction is controlled to a marked extent by the magnitude of the potential existing across the 1unit. Devices having these general properties are often termed varistors, and will be so designated in this description. In a tested embodiment of the invention, these devices 52, 56 were dry-disc copper oxide type of varistors. Capacitors 58, 54 act to neutralize the capacities that are inherent in this type of varistor, and together with these inherent capacities form a balanced capacitive bridge arrangement. The values of capacitors 58, 54 will, of course, depend upon the inherent capacitance of the actual varistors that may be used. Conductors 62, 64 are connected to the mid-points of the windings of transformers 50, 60 and serve to impress across the varistors 52, 56, a varying potential which is derived in a manner which will now be described.

Capacitors 6I, 92, 93 and inductor S5 comprise a band-pass filter which has minimum attenuation at a frequency corresponding to the middle of the message signal band. The attenuation of this filter at frequencies outside the band of message signal frequencies is relatively high. The' message signal frequencies are passed through a transformer 94 to a full wave rectifier 95 to develop across potentiometer 84 and filter vcapacitor 96 a unidirectional voltage which is indicative of the presence of and the strength of message signal frequencies in the output of the radio receiver. The polarity of this rectified voltage is opposite to the polarity of the voltage that is' developed across potentiometer 84 by the anodeacta-ccs cathode current of `the electron discharge device, specifically -a pentode, 85, which also flows through this potentiometer. Conductors v62, -64 are connectedacross potentiometer Ell -i-n such fashion that they apply to the varistors 52, Aliti a potential which is the algebraic sum of the potentials developed Yacross potentiometer 181| as described. vIt might be stated at thi-s point that this last described circuit branch, comprising the full wave rectifier 95, is provided in order to desensitize the noise detector 2|) during periods when lthe 4level of the message signals at the transmitting station are exceedingly high. The reason for :this will be developed in Yconnection with the description of the operation of the circui-t. 'Thus the transmission loss through. -the variable 'loss :circuit is controlled by the potential that is applied over conductors 62, -64 to the vari-sters 52, 5E in this circuit.

The seconda-ry wind-ing of transformer 6|! is terminated by potentiometer 59. The control grid of .an electron discharge device, specifically, a pentode, 6| is connected Ato the movable arm of :potentiometer 5:9, and `this device comprises the first .stage of .a .two-.stage amplifier which includes .the pentodes 6|, .66. This amplifier may be .in .accordance with conventional design. Its circuit vconstants are so. chosen that it has a substantialiy .uniform amplification .characteristic over a range which includes any desired frequencies above .and below the message signal frequencies. The output vof this amplifier is connected through a transformer 68 to a .full wave rectier .circuit comprising a double diode l2, terminating resistors 7.0, 1| and load resistor l5. The output of this rectifier is passed through an energy yStorage and filter arrangement comprising load resistor l5., a capacitor ld, and the resistor-capacitor combination. 76, The time constants of the rectier and storage .circuits are chosen such that Aa finite time is required for the energy contained in sudden sharp bursts of noise to .develop full potential across .capacitor 1.4. In one tested embodiment, .a suitable time constant for this circuit was found to be about 100 microseconds. After a potential has been developed across `capacity 'i4 by a surge of energy, thiscondenser will discharge partly through resistor 'l5 and partly through the resistor-capacitor combination 1.6, 77|, This latter combination forms a iilter circuit which prevents undesirably sudden changes .of potential from ,accumulating across capacitor Tl. The discharge time constant of capacitor 'I4 is preferably relatively long with respect -to its charging time constant, and in one tested embodiment o-f the invention .a discharge time of about l0 milliseconds was found to be satisfactory. The successful practice of the invention is, of cou-rse, not limited to the use of the above-mentioned time constants. Any other suitable time constants may be used with equal facility. In general, it is believed that the relationship of the charging and discharging rates should be correlated with limiting factors in other portions of the communication system; for example, such factors as the cut-off characteristics of the signal channel, and the response of, or appreciation period, of a limiting unit in the utilization process. In an aural communication system, wherein the highest frequency of the message signal is restricted to some definite value, and in which some element, such as the ear, has a fairly definite minimum appreciation period; it might be desirable to restrict the charging period to a value suitably less than one-half thecyclic period vof the highest usable signal fre-A quency, and to arrange the discharge period at a value suitably less than the kminimum appreciation period of the ear. Such an arrangement imparts to the detected interference components a weighting fac-tor that is variable with frequency, and, which causes the control efect of these `detected components to vary in a manner that lis similar to that in which the impairment effect `of noise components is Vobserved to vary in the utilization circuit. At the same time, the response of the detection device is fast enough to permit the proper evaluation, or appreciation, of undesirable `quantities of interference .energy in a :shorter period than is required by the limiting receptive member for a similar appraisal. The control eiectrode of pentode is connected to the common point of resistor-.capacitor combination i6., 77|.. The cathode circuit of pentode 80 includes an .adjustable resistor V82 and the potentiometer .34, across which are connected conductors 62, 64. Anode power supply for pentode 8B is secured from the respective longitudinal balancing `circuit 24 (Fig. l3) at the control terminal poi-nt or site. This potential is transmitted to the location of noise detector 2|! over the upper conductor of conductor .pair 313. The lower condoctor of conductor pair 3,3 forms the return path for this circuit and is connected to the anode-cathode circuit of pentode 8B at the lower end `of cathode load potentiometer .84.. Capacitor 855 joins the upper and lower sections of the secondary Winding of transformer 3| to forma conductive path for the voice frequency signal currents from receiver t2, and to block the direct current that i-s flowing in the upper and lower conductors of pai-r 33.

A similar blocking function is performed by capacitor ill-,I at the longitudinal balancing unit end of conductor pair 33, as is shown in Fig. 3. Transformer |09 is inserted in the circuit .of conductor 'pair 33, and, together with capacitor IDI, forms a blocking circuit for the direct current derived from potential .source '|20 through resistor |02. The anodeecathode circuit of pentode 80 (Fig. 29 includes potential source |20, load resistor |32, the upper conductor of conductor pair 33., resistors 82 v81|, the lower con ductor of lconductor pair 33, resistor |03., a p0rtion ofr voltage dividing potentiometer |04 to the grounded terminal of source 20.

Voltage dividing potentiometer L04 provides a conyenient .and ready means .of adjusting the potential of .the cathode of pentode 80. This potentiometer provides a lmeans for imparting a small, xed adustment to the unidirectional signal voltage as it is Obtained from the noise detector 20, and permits the .proper coordination of these voltages from the various noise detector units `of a selecting system. Resistors |132 Aand |03 may be .approximately equal and y0f any vsuitable value consistent with the length of conductor pair 33 and the .characteristics of Dentode .80.

With respect now to the longitudinal currents balancing circuit or unit 24 anode potential for balancing electron discharge device or pentode |09 is supplied from potential source |20 through anode vload resistor |85. Resistors Hit, |08, potentiometer v|1|l| and .anode load resistor |05 formv .a volta-ge dividing circuit. The movable arm of potentiometer |01 is connected through a coupling capacitor ||l5 to the Vcontrol electrode of pentode |09. Values for these components and 'the dynamic characteristic of pentode I-f09 are coordinated such that, the amount of volt' age change that is applied to the control grid of pentode |09 is just sucient to counteract in its anode circuit the voltage change that occurs at the junction of resistors |02, |08. For example, if the potential at the junction of these latter resistors changes by one Volt in a positive direction, the potential at the anode of pentode |09 will change by an equal amount in the opposite direction. Because of the large amount of voltage feedback present in the anodecontrol grid coupling circuit, the transmission equivalent between the junction of resistors |02, |08 and the anode of pentode |69 is determined primarily by the voltage dividing circuit comprising resistors |06, |01 and |08, and is relatively independent of changes in the other parts of the circuit. Resistors ||6, ||8 form a voltage dividing network for the proper biasing of the control electrode of pentode |09. The anode circuit of pentode |63 is connected through coupling capacitor H to two voltage dividing circuits, one of whichincludes equal valued resistors and ||2, the other of Which includes equal valued resistors ||3, ||'4. The value of these resistors may, but need not be, the same so long as the paired resistors are equal valued. Conductors |22 and |25 are connected at one end of each to the junctions of resistors I, i2 and H3, I4 respectively. The other ends of these conductors are connected to the input circuits of the selecting and threshold branches, respectively, of receiver selector |6 (Fig. 4).

The noise-indicating signal on conductor |22 is supplied to the input of the channel selection branch of selector |6 (Fig. 4) which branch includes an electron discharge device, specifically, a triode |34. Resistors |26 0r |28 and capacitor |32 constitute an integrating network for adding a predetermined delay to the voltage changes that are supplied to the control electrode of the triode |34. Whether resistor |26 or resistor |28 forms a component of this integrating network depends upon Whether relay |68 is operated. Resistor |28 is smaller than resistor |26 by any desirable amount in order to provide a faster selection by triode |34 at the beginning of a transmission interval.

In its non-conduction state, the anodecathode circuit of triode |34 includes source |36 of anode potential, conductor |37, the winding of relay |38, the space-discharge path of triode |34, conductor |50, contact of relay |42, conductor |44, cathode-biasing resistor |46 and a source |48 of negative potential, the positive terminal of which is connected through a common ground connection to the negative terminal 0f source |36. A source |70 of negative potential is connected over conductor Ulli to the winding of relay |42, and meets operating ground over the contact of relay |38 when this latter relay is operated. Resistors |50, |52 are connected over conductors H34, |45 to form a voltage di- Vider across a biasing source and its lter capacitor |55. The cathode of triode |34; is connected to the junction of these resistors. When relay |42 is operated, in a manner which will be presently described, the message signal conductor pair 33 is connected over contacts 2 and 3 of this relay and conductor pair |43 to contacts 5 and 6 of relay |68 in the threshold branch of the circuit. Operation of this latter relay connects the signal output of the selected receiver 2 (Fig. 2) to the utilizationcircuit i8.

1'()l The operation of relay |68 also supplies ground over contact 7 and lead |29 to a signal lamp in the terminating switchboard of utilization circuit I8, when provided, to indicate that vsome one of the receivers I2 is producing a message signal of usable quality.

The unidirectional voltage signal from each longitudinal balancing unit 24 is supplied over the respective conductor |24, |24', etc. to a unidirectional device |56, |56', etc., in the threshold branch of the circuit. The cathodes of these devices are connected in parallel and their cir-l cuit includes load resistors |58, |60 and a Dortion of potentiometer |62. The control electrode of an electron discharge device or triode |64 is connected to the common point of resistors |53, |60. The lower end of resistor |60 is connected to the movable contact of potentiometer |62, which forms a voltage divider across the source |48 of negative potential. The anode-cathode circuit of triode |64 includes the source |36 of positive potential, the windingv of a relay |66, the space discharge path of tri.- ode |64 and a ground connection which is common to the source |36. The operation of relay |65 connects operating ground over its Contact to one terminal of the winding of relay |68, the

other terminal of which is connected to thev source |10 of negative potential.

It will be noted from Fig. 4 of the drawing that,

in this described embodiment of the invention, se..

lector |6 comprises four parallel branches, or channels.

each of these channels are substantially identical selecting.

with those of the rst, or upper, channel which.

have been described. Like circuit elements have been assigned the same reference numeral in each channel, and either a single, a double, or a triple prime exponent has been added to the numeral,

to properly identify the element invits branch, or channel location. Most of the circuit details of the intermediate branches, or channels, of selector |6 have been omitted in the interest of sim. plicity, and the presence of these branches is indicated by the yblock diagrammatic representations.

Description of operation The operation of the selecting system of this. invention, the structure or circuit arrangement of which has just been described, can best be unY derstood from the following description of the` operation of the arrangement shown in Figs. 2, 3 and 4. Assume first that no receiver of the systern, as exemplied by receiver l2 (Fig. 2), is receiving a usable train of message signal waves.,

Under these circumstances, the noise energy in the output circuit of receiver i2 will ordinarily be at a relatively high level. In a manner which will be presently explained, the bias voltage on the control electrode of pentode 86 in noise detector 26 will be at a relatively high negative value, and the current flowing in its anode-l cathode path and in conductor pair 33 will be low. The voltage drop across resistor |63 (Fig. 3)

will be small, and the potential that is supplied to conductors |22, |24 by the potentiometer circuits comprising resistors H2, H3, ||4 will be low. The actual value of this potential onA 33, over contacts 2, 3 of relay |42', to-intercon` The circuit details ofvv 1.1'. necting pair |43, the circuit of which is open at contacts and 5 of relay i623. The potential supplied over conductor |24 to diode |56 in the threshold branch of selector l5 will be low. and the voltage that is developed across voltage divider |58, |50 will also be low. The portion of this potential that is supplied to the control grid of triode |64 will be insufcient to overcome the negative bias that is derived by its control electrode from potentiometer |62. Under these circumstances, triode |55 will not conduct current, its anode load relay E55 will not be operated, threshold relay |58 will not be connected to groundfrom ther contact of relay |56 and will remain unoperated, and the circuit from con-y ductor pai-r M3v to the utilization circuit I8 willl be broken. or open at contacts 5 and 6v of relay |58. Therefore, notwithstandingr that the upper selecting branch of selector i6 may have functioned to connect the signal output of receiver l2 to interconnecting pair |43, the output of this receiver isnot connected to utilization circuit i8 because the threshold circuit has adjudged the received noise signal to be representative of output signals that are less than the just usable, or minimum, commercial grade. This just-usable or minimum value signal is a predetermined quantity, and may be changed by varying the position of' the movable arm of potentiometer |52. Now assume that a usable grade of audio message signal exists in the output of receiver i2. level of energy in the output circuit of the receiver is controlled by the level of modulation at the transmitter, the noise energy in the output circuit of the receiver will be reduced from its former high value by some unspecified amount. revised output energy is treated in the same manner as was the previous noise energy. A portion of it is transmitted through attenuator 29 and transformer 3| to conductor pair 33, and over this conductor pair to contacts 2 and 3 of relay |42 in receiver selector i6. Assume for purposes of this explanation that, at this time, this relay is unoperated and contacts 2 and 3 are open. A second portion of the signal output from receiver |2 is supplied over conductor 3E to the frequency sensitive network comprising capacitor 38 and resistor 40 and voltage dividing resistors 42, d4. The transmission loss through this network varies inversely with frequency. The network has two output circuits,l one of which is connected tothe band-pass filter comprising inductor 99 and capacitors 9|, 92. this filter is supplied through transformer 94 to full-wave rectifier 55. The rectied output voltage is applied over conductors 52, 6'4 to the variable loss circuit comprising varistors 52,. 5G, and tends to control the loss through. this circuit by controlling the impedances of the-varistors. The second output from the frequency sensitive. network 38, El is applied over conductor 45 to theA input of band-reject filter 48, the characteristics of which are such that it passes frequencies both above and below, but not within the band of message signal frequencies. Because of the frequency discriminating action of network 38, 45, the higher frequency noise components are transmitted to band-reject filter 48 with greater facility than are the low frequency components. As was previously stated this arrangement is helpful in evaluating the interfering effect of unwanted radiations which may be modulated by low frequency components. The noise energy that is passed by lter 48 is transmitted through the variable loss circuit comprising transformers 55,

60 and varistors 52, 56 to the amplifier comprising Because thev The message-signal output from.

devices 6|, 66. This ampliiied noise energy is; rectied in the full-wave rectifier circuit com:- prising the double diode l2, balancing resistors 1G.. H and load resistor 15, together with its filter which includes capacitors 14, ll and resistor l5. Because of the limiting action in receiver |2, less, noise: energy will now be rectified than was previously the case, and the potential across capacitor 'H Will be reduced. yl-he negative. potential at the control grid electrode of pentode will now be less than in our previously assumed case, where no message signal was being received. The anode-cathode current in this pentode will now be increased, and will provide a greater voltage across potentiometer 84 than heretofore existed. The potentials that are generated across this po.- tentiometer by the passage through it of cathode current from pentode 8|) and the rectified currentI from rectifier are opposed in polarity. The net potential is supplied over conductors 62 55 to the previously-mentioned variable loss circuit comprising varistors 52, 55 where it controls the transmission loss through this circuit .by affecting the characteristics of these varistors.

The action of this feedback arrangement is to linearize the relationship between the current flowing in the anode-cathode circuit of pentode. 8i), with respect to logarithmic variations in the level of the noise input to detector 293. This action comes about because the magnitude of the anode-cathode current of pentode 8!) is anv inverse function of the level of the rectified noise energy. As the noise level is decreased, the cath.- ode current of pentode 80 and the potentialacross. potentiometer 82 are increased. This increased voltage is fed back over conductors 62, 82- to the Variable loss circuit where it decreases the transmission loss by controlling the characteristics of varistors 52, 56. This action tends to increase the rectified noise energy, and thereby reduce the cathode current of. pentode 8U, and the potential across potentiometer 84. The net effect of this arrangement is to produce a stabilized system having the desired characteristic, such as is shown by the idealized curve |72 of Fig. 5. Curve |12 shows the manner in which the anode-cathode current in pentode 8i) may be changed by logarithmic'variations in the level of the noise energy that is applied to the input of detector 20 when sufficient voltage is fed back to control varistors 52, 56 possessing suitable current-voltage characteristics. If no feedback arrangement, or its equivalent, were used the input-output characteristic of detector 25 would be similar to that shown by curve |14 of this figure. It will be noted that this latter curve varies considerably from the more desirable logarithmic characteristic and that its operation is restricted to a rela'- tively small range of input levels. The departure may be great enough that signals having a suitable or acceptable noise ratio would be rejected. From the described action of detector 28, it will be appreciated that this unit produces in its anode-cathode circuit, and, therefore, in the direct current circuit which includes conductor pair 33, a current the magnitude of which increases linearly as the logarithmic relation between the signal and noise in the output of receiver l2 is increased. Therefore, for low noise values this current will be at its maximum values. Potentiometer 84 is made adjustable to permit control. of the amount of this fed back voltage so that essentially th-e same input-output characteristic may be obtained from each noise detector 20 in the selecting system.

Because non-linearity generally exists in thel 13 transmission path between the input to a radio transmitter and the signal output of its associated receiver, there results a distortion product or products whenever a message signal is transmitted. This distortion product may not be appreciable under ordinary circumstances, but it becomes more prominent as the input signal attains an extraordinary high level. This corresponds to a high level of modulation, which produces a loud or high level output signal. Many of these distortion products fall in the frequency range above and below the signal frequency band, and are thus passed by the band-reject filter 48. .Since they are passed lby this iilter, the detector circuit 20 would normally treat them as indications of objectionable noise conditions, if it were not for the operation of that portion of the circuit that comprises full-wave rectiiier 95. This portion of the circuit receives some of the audio signal energy from the frequency sensitive network 38, 40 and produces from it a unidirectional voltage, the

magnitude of which is indicative of the level of f the message signal, and the polarity of which is opposite to the polarity of the voltage generated across cathode potentiometer 84 by the flow of cathode current in pentode 80. Being thus poled, this rectified voltage acts on varistors 52, 56 during period of high message signals to increase the transmission loss through these units, and thus to momentarily incapacitate noise detector 20. Through this action, the noise detector is prevented from mistaking distortion products, which arise from otherwise desirable message signals, as objectionable interfering noise currents.

It will be noted that detector 2 0 is isolated from direct ground connection, and that the anode power supply for pentode 80 is derived over the line 33 from the associated longitudinal balancing unit 24 (Fig. 3) 'which may be, and probably will be located at a considerable distance from detector 20. None of the circuit elements associated with the line and pentode 80 has any direct current connection to the ground at the receiver location, the potential of all points in that part of the circuit being determined with respect to the ground potential at the control terminal. This prevents a difference in ground potential between any receiver station and the control terminal from altering and thus ialsifying the indication signal.

The anode-cathode current of pentode 80, the magnitude of which is indicative of the ratio of the signal and noise in the output circuit of receiver I2, flows through conductor pair 33 and, therefore, is subject to any induced interfering voltages. Since this circuit is isolated from ground potential, these interfering voltages will be induced in equal amounts of the same polarity in each of the conductors of the pair.

The action of the longitudinal balancing unit 24-is such that the eect of these induced voltages is eliminated. Positive potential is supplied from source |20, through anode load resistor |02, over one conductor of pair 33, to the anode of pentode 80 in detector 20 (Fig. 2). This tubes cathode current flows through potentiometer 84,

'one conductor of pair 33, through resistor |03 and biasing resistor |04 to ground. Therefore, as the anode-cathode current through pentode 80 (Fig. 2) increases, the potential with respect to ig'round at the lower end of resistor |02 is decreased and at the upper end of cathode resistor |03 is increased. These resistors are substantially Y equalin value and, therefore, these changes in 4potential are equal and opposite.

A portion of the potential change across anode resistor |02 is applied `through the movable contact arm of potentiometer |01 and coupling capacitor ||5 to the control electrode of pentode |09. This voltage produces in the anode circuit of pentode |09 a voltage change which is equal in amplitude and opposite in polarity to the change across resistor |02. This equal and opposite change is coupled through coupling capacitor 0 to the parallel voltage dividing circuits comprising resistors I, ||2, and H3, H4, Since the voltage at each end of the dividers is changed by the same amount, the potential of conductors |22, |24 with respect to ground is changed by an equal amount. If, however, interference voltages of the longitudinal type are induced on conducting pair 33, there will appear at the lower and upper terminals of resistors |02, |03, respectively, potential changes which are equal and of the same polarity. Under these circumstances, when a portion of the change that appears at the lower terminal of resistor 02 is applied through the movable contact of potentiometer |01 and coupling capacitor ||5 to the control electrode of pentode |09, there appears in the anode circuit of this pentode a potential change which is equal in value but opposite in polarity to the potential change at the upper end of resistor |03. Since equal but opposed changes are made at the two ends of the voltage dividers, the potential on connecting circuits |22, |24 remains unchanged, and the interfering effect of the induced longitudinal voltage is eliminated.

The voltage on conductor |24 controls the operation of the threshold branch of selector I6 (Fig, 4) while that supplied over conductor |22 controls the operation of the receiver switching or selecting branchof the selector unit. In the following explanation of the operation of receiver selector I6 it will be understood that, although only one longitudinal balancing unit 24 (Fig. 3) is shown as providing unidirectional voltage signals to conductors |22, |24 of selector I6, similar signals are supplied over conductors |22', I 24 etc. to the remaining switching, or selecting, and threshold branches of this circuit. These voltage signals are obtained from the noise detectors 20', 20, 20 and longitudinal balancing units 24', 24", 24 that are associated with the other receivers of the communication system. It will also be understood that the details of these circuits, although they are not shown on Figs. 2 and 3, are to be taken to be substantially the same as the details there shown for that one selection branch. The voltage on conductors |22, |24 is, as was stated, a single-ended signal, the magnitude of which varies inversely as the level of the noise energy varies in the output of the respective receiver. In our assumed example, the signal in the output circuit of receiver |2 is changed from one which consisted almost entirely of noise energy to one in which the message signal energy is dominant, and a relatively small amount of noise energy is included. The voltage signal on conductors |22, |24 is accordingly changed from a low voltage value to a relatively high voltage value. As the voltage on conductor |24 is increased, the negative bias on the control electrode of threshold tube |64 is counteracted by the potential developed across the rectifier load comprising resistors 53, i60, and the potential of this electrode is raised to a value where the resulting anode current ilow is sufficient to operate relay 66. This condition corresponds to the reception of a usable or commercial grade of signal by receiver I2. It will be readily seen that this same action would occur if this ,com-

mercially usable signal had been received by anyone of the other receivers I2', |2" or |2"'. Relay connects ground over its contact, through the winding of threshold relay |68, to potential source and causes this latter relay to operate, which action opens contacts 2, 3 and 4 and closes the contacts 5 and 6 of this relay. The closure of contacts 5 and 0 connects the output of a selected receiver, in this assumed case receiver |2, to the utilization circuit |8.

The manner in which receiver i2 is selected and the remainder of the receivers are rejected by the selection branch of selector i6 will now be explained. It was assumed that thel anode current flowing in triode |34 was insuflicient to hold its anode load relay |38 in the operated condition. When the voltage signal on conductor |22 changes from a relatively low value to a relatively high value, which value is in excess of any voltage signal that exists on incoming conductors |22', |22, |22"', triode |34 is caused to increase its anode-cathode current to a value that is suflicient to operate relay |38. Relay |38 connects ground over its contact to the winding of relay |42. The operation of relay |42 opens the shortcircuiting connection across resistor |52, provided by normally closed contact of relay |42, and closes over its contacts 2 and 3 the connection from conductor pair 33 to interconnecting pair |43. Since pair |43 is closed through contacts 5 and 6 of relay |68 to the utilization circuit |8, the output of receiver I2 is connected to this latter circuit.

It will be noted that resistor |46 is included in the cathode circuit of each triode |34, |34', etc. The voltage that is developed across this resistor by the combined current flow in all triodes |34, |34', etc. exerts a degenerative feed-back effect which tends to bias the cathode of each tube positive with respect to the potential of its control electrode. Therefore, if the same potential exists on each control grid electrode, theY cathode of each triode will be at the same potential, however, if the potential of the control electrode of one triode exceeds that of the control electrode of any other triode, the current flow through the triode having the highest potential on its control electrode will establish the potential at all of the cathode electrodes. Therefore, as the current flow in triode |34 increases because of the increased potential on its control electrode, its cathode current flowing through the common load resistor increases the potential of its own cathode electrode and that of all other connected cathode electrodes. If the potential increase is suffi'ciently large the entire anode-cathode current flow will be transferred to triode |34, and the remainder of the triodes |34', |34", etc. will cease conduction. This condition will continue until some one of the longitudinal balancing units 24', 24, etc. supplies to its associated selection branch of selector I6 a potential which is substantially equal to that existing on the control grid electrode of triode |34 in the upper branch. If such a condition prevailed, receiver selector i6 might connect the output of two receivers, for example, receiver |2 and l2' to the utilization circuit I8, or it might be subjected to undesirable indecision by switching from the output of one to'that of another of the receivers as the noise energy in the output circuits of these receivers fluctuates one above the other by a slight amount. To prevent these conditions, a preferential biasing arrangement is provided such that when a usable'signal has been chosen by selector l5, it

will continue to be chosen until it is supplanted by a more desirable signal which exceeds the chosen signal by a predetermined amount. This preferential biasing feature is brought into action when relay |42 opens the circuit which was formerly closed over its contact This action removes the short-circuit connection which formerly existed across resistor |52, and produces across the combined resistors |50, |52 a voltage drop which is equal to the potential of source |54. The effect of this action is to lower the potential of the cathode electrode of triode |34 with respect to the potential of the remaining cathode electrodes by a predetermined amount which is controlled by the relative values of resistors |50, |52 and the potential of source |54, Under these circumstances then, before any other branch of selector IS can connect the output oi its associated receiver to utilization circuit l, and in so doing bias triode |34 of the upper branch to its non-conduction state, the potential on the control grid electrode of its associated discharge device must exceed the potential on the control grid electrode of triode |34 by at least the amount of the voltage drop across resistor |52.

Each electron discharge device or vacuum tube |34, |34', etc., of the selector tube array is preferably chosen for a high transconductance wherein a relatively small change in grid-cathode potential difference will produce a relatively large change in anode-cathode circuit current. As already referred to hereinabove, the circuits associated with the electrodes of these tubes include feedback by virture of the common cathode resistor |46, to establish an anode-cathode current in that one of the tube array whose control grid has received the highest-valued input signal, that is, input signal respective the best merit-indicating signal. This anode-cathode current remains substantially constant even though the potential applied to the control grid of such one tube varies or changes over a range corresponding to a wide variation of input; assuming, of course, that the range of variation of the potential applied to the control grids of the other tubes during such period is at all times less than the then applied potential on the control grid of the one tube, taking into consideration, also the preferential bias provided by the resistors |52, |52', etc. However, when the control grid potential on one of the other tubes is such that current is caused to flow in the anode-cathode circuit of such second tube, then a small variation in the potential of either such one tube or such second tube produces large changes of current in their respective anode-cathode circuits. In a tested embodiment of such a tube array, the devices |34, |34', etc. were R. C. A. type GSL'IGT; the resistor |46 was of 100,000 ohms; resistors |50, |50', etc., and resistors |52, |52', etc., were of 1,000 ohms each; the relays |30, |38', etc., were chosen and adjusted for operation on approximately .7 milliampere and for release Ion approximately .4 milliampere; cathode-anode circuit current through the cathode resistor |46 varied between approximately 1.5 and 1.9 milliampere.

For descriptive purposes, let it now be assumed that there is received on conductors |22"' and |24"', a signal having an. amplitude that is greater than that of the signal on conductors |22, |24 by more than the voltage drop across resistor |52. rThe main anode-cathode conductionl path will be transferred from the upper selection branch, Vwhich includes triode |34, to the lower selection branch, which includes triode |34"",

because of the degenerative bias that the increased anode-cathode -current in triode |34'" imposes upon the control electrode of triode |34. Under these circumstances, triode |34 will cease conducting, relays |38 and |42 will release, and, in so doing, open the connection between conductor pairs 33 and |43, and restore the short circuiting connection across resistor |52. Simultaneously with this action, relays |33 and |42" will be operated to connect conductor pair 33 to conductor pair |43, and to open the short circuiting connection around resistor I52" over contact I of relay |42, to add the preferential bias voltage across resistor IEZ" in favor of the lower selecting branch. The lower selecting branch will continue to connect the output of its associated receiver |2 to the utilization circuit I8 as long as the merit indicating signal provided by its longitudinal balancing unit 24 continues to exceed that supplied by any of the other associated balancing units. Since the potential that existed at the control electrode of triode |54 in the threshold branch of selector I6 was adequate to provide sumcient anode-cathode current in this triode to hold operated the'anode load relay |66, the increased signal on conductor |24 causes no added effect.

If the voltage that is derived by the threshold circuit from conductors |24, |24', etc. is at any time decreased to a value where the potential on the control electrode of tube |64 is insufficient to hold relay Ie` operated, the -connection between conductor pair E43 and utilization circuit I8 will be broken; notwithstanding that some one of the selecting branches of receiver selector |6 may 'be operating to connect the output of its associated receiver to conductor pair |43, and to exclude the output of all other receivers in the system. If this condition exists for an undesired interval, and it is desired to accept any one of whatever understandable or decipherable receiver outputs are actually present, appropriate adjustment at the control terminal of the potentiometer |62 will effect a shift downwardly of the operating level of the device |64, and render what would not be acceptable with the initial setting of thepotentiometer, connectable through to the utilization circuit. Of course, if instant radio transmission conditions or the presence of extraneous industrial interference should preclude any character of usable or understandable signal being produced in the receiver output circuits, use of the system necessarily would await upon the clearing of such conditions.

The invention has been described as being incorporated in a receiver selecting system that is arranged to chose between the output signals of four receivers of the same message signal wave. It should be evident that this is only one embodiment of the invention, and that variations of this arrangement which do not depart from the spirit and scope of the invention will occur to those skilled in the art. .Although the invention has been described as being incorporated in a radio communication system, it should be appreciated that it may be applied with equal facility in a variety of different types of selecting and switching arrangement in which the relative desirability of the various units or circuits in indicated by an appropriate derived or representative electrical factor, for example, a unidirectional voltage signal.

What is claimed is:

1. In a radio communication system, a receiver of radio frequency message signals adapted to translate received signals into audio frequency message signals, means at the receiver location for selecting from the output of said receiver a portion of the energy in the frequency spectrum immediately above and below, but not within, the band of frequencies of said audio frequency message signals and for deriving from said selected portion a control signal the amplitude of which is indicative of the level of the noise energies in the selected portion, and means responsive to the level of the auido frequency message wave for controlling the amplitude of the derived signal during a period when the audio frequency message signal waves are at an unusually high level.

2. In a radio communication system comprising a receiver of radio frequency message signals adapted to translate-its received signal into an audio frequency message signal, means at the receiver location for selecting from the output of the receiver a portion of the energy in the frequency spectrum immediately above and below, but notwithin, the band of frequencies of said audio frequency message signals and for deriving from said selected energy portion a control signal the amplitude of which increases in logarithmic relation to decreases in the amount of noise energy in the selected portion, and means for momentarily increasing the amplitude of said derived control signal during intervals when the received audio frequency message signal is at an unusually high level. y

3. In a radio communication system comprising a receiver of radio frequency signal wave adapted to translate Vsaid received signals into audio frequency message signals, means at the receiver location for selecting from the output thereof a portion ofthe energy in the frequency spectrum above and below but not within the band of frequencies of said audio frequency message signals and for 'deriving from said selected energy portion a control signal the amplitude of which is indicative of the level of the noise energies in the selected portion, sai-d means comprising a variable transmission lossl circuit through which said selected noise energies are transmitted, meansfor controllably varying the transmission loss through said circuit in direct accordance with the level of the noise energies in said portion, and changing the transmission loss through said variable circuit in' direct relation to the level of the received audio frequency message signal.

4:. An electrical circuit for deriving an electrical quantity representative of the interference energy accompanying a band of signal frequencies, that comprises a transmission path for said signal band and accompanying interference energy, means for further transmitting only wave frequencies above and below those of said signal band, a network of variable transmission loss characteristic for receiving the output of said means, means for deriving a control potential from the output of said network, means responsive to said control potential for generating said representative electrical quantity, and means for generating a control potential determined by the central frequency portion of the signal band and for applying such latter-mentioned control potential to said network to adjust the transmission loss of said network to a further degree with respect to the output of said first-mentioned means.

5. The electrical circuit of claim 4 in which means for controllablyl 159 said last-mentioned meansl comprises' a selective: electrical lter, rectifyingf. means connected with the output of saidrlte'r. and-means fon applying the output of; the rectifyingf. means to said network.

6. The electrical circuit of claim in which the control potential' generated by the last-men.- tioned means increases the transmission loss. in said network during. anlplitudel levels: inthe signal band above a preassigned level..

7. An electrical circuit for deriving a, variable electrical quantityrepresentative, or the varying interference wave. content of; av band; of.' signal frequencies, that comprisesl a3. transmission path. for said signal band'. andv interierence` content, a band elimination lter for transmitting only wave4 frequencies outsider of.: said: signal band. a variable loss network forreceving; the output ot said. iilter, means for adjusting? the'transmissioniloss eiected by said loss network rectifying. means for converting the output of saidAv loss network into a controlv potential, means' variable in: in1- pedancein response to'y said controlf potential and having an output circuit in which said repre-Y sentative electrical' quantityv is-thereby'generated', a feedback circuit betweerr said. variable impedance means and said network and;V including. said'. means for adjusting the transmission loss of said loss network such that' the generated electrical.

quantity bearsa substantially straight linerelation with respect to the logarithm of: the electrical input to the loss network means,4 in the output circuit of said variable impedance means to adjust the amount of feedback, an electrical lter for transmitting wave; frequencies within the signal band and having minimum attenuation in the midportion oi the signal band and relatively high attenuation outside oi'V it, and means for converting the output of said electrical lter into direct current and for applying the latter to said loss network. to increase the latters transmission loss with respect to. the output ofv said band elimination. filter.

8. An electrical circuit. for deriving an electrical quantity representative of the interference energy accompanying a band of signal frequencies, which compirses al transmission path tor said signal band andaccompa-nying, interference energy, means for further transmitting only' wave frequencies in a preassigned frequency relation to said signal band, a network of variable transmission loss characteristic for receiving the output of said means, means for' deriving a control potential from the output ofY said: network, saidv means including an energy translating and storagermeans: having' a charging time which. is short in comparison. to its discharging time, means' responsive to said control', potential for generating said representative electrical quantity, and means for applyingy a portion oi said generated quantityl to said network to control 'the transmissionloss introduced by the latter in accordance withv variations in the magnitude of the generated electrical quantity;

9. The electrical circuit of claim 3- in which said means for deriving a control potential com prises rectifying and energy storage means having. a charging, time' constant and a discharging time constant, the'relation between said constants being substantially in the order of' 1 to 100, respectively.

10. The electrical circuit of claim 8 in which said means for deriving a control potential comprises a unidirectional conducting device and a capacitive-resistive energy storage device, said` unidirectional and said storage devices 1oeing so arranged. that the energy discharge time oi said devices isrsulcstantially times as great as the energy charging time of said devices.

11. The electrical circuit of claim 3 in which said means for deriving a control potential cc.; prises recti'iying and energy storage means having a charging time period which is slightly lessv than one-half the cyclic period of the highest signal component in said band of signal frequencies, and the discharging time period of which is severalY times greater than the charging period.

WILLIAM R. YOUNG, Ju.

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